1. Field of the Disclosure
The disclosure is related to the field of electromagnetic induction well logging for estimating a property of earth formations penetrated by a wellbore.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 (“the '671 patent”) issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along an instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
Resistivity tools, referred to as deep-looking electromagnetic tools, are used to determine properties of earth formation at distances from tens to hundreds of meters (ultra-deep scale) from the borehole. Such tools are typically limited to high resistivity formations and are further constrained by the power available downhole.
Induction measurements in frequency domain are widely used in LWD/wireline formation evaluation (FE) and geosteering. Acquiring measurements at more than one frequency allows one to apply a multi-frequency focusing technique (MFF) which efficiently eliminates near-borehole effects, increases depth of investigation, and simplifies the dependence of tool responses to formation parameters. The latter is especially important in interpreting multi-component data in deviated wells where the complexity of the tool responses often precludes from obtaining fast and accurate results. The MFF applications with conventional and multi-component measurements are described in numerous patents, papers, and reports and include but are not limited to:                Determining formation resistivities and resistivity anisotropy in isotropic, transverse isotropic (TI) and bi-axially anisotropic models;        Determining formation dips and azimuths in anisotropic formations;        Using the MFF technique in LWD FE applications and geosteering in the presence of a drill pipe;        MFF deep geosteering based on distance to bed processing and/or processing for formation dip and azimuth;        Processing and corrections for fracture.See, for example U.S. Pat. No. 7,027,922 to Bespalov et al., U.S. Pat. No. 7,031,839 to Tabarovsky et al., U.S. Pat. No. 6,574,562 to Tabarovsky et al., and U.S. Pat. No. 7,392,137 to Tabarovsky et al., all having the same assignee as the present document and the contents of which are incorporated herein by reference.        
At the ultra-deep scale, a technology may be employed based on transient field behavior. The transient electromagnetic field method is widely used in surface geophysics. Examples of transient technology are seen, for example, in Kaufman et al., 1983, “Frequency and transient soundings”, Elsevier Science.; Sidorov et al., 1969, “Geophysical surveys with near zone transient EM.” Published by NVIGG, Saratov, Russia.; and U.S. Pat. No. 7,027,922 to Bespalov et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. Bespalov teaches the use of differential or integral filtering of the latter part of a transient signal to estimate formation properties in the presence of a mandrel with finite conductivity.
Multi-frequency focusing (MFF) is an efficient way of increasing depth of investigation for electromagnetic logging tools. It is being successfully used in wireline applications, for example, in processing and interpretation of multi-component measurement devices. An example of such a device is the 3DExplorer® (3DEX®) induction logging instrument of Baker Hughes. In the 3DEX® instrument, three transmitters are placed axially on a tool mandrel and induce magnetic fields in three mutually orthogonal spatial directions: x, y, and z. The z-axis is chosen to be along the longitudinal axis of the tool, and the x-axis and y-axis are mutually perpendicular lying in the plane transverse to the z-axis. Three receivers, Rx, Rz, and Ry, are aligned along the orthogonal system defined by the transmitters. Measurements can be made for the corresponding magnetic fields Hxx, Hzz, and Hyy, as well as cross-components, Hxy and Hxz. The 3DEX® is operable in single frequency and multiple frequency modes.
MFF is based on specific assumptions regarding the frequency-dependence of electromagnetic field in the frequency domain. For MWD tools mounted on metal mandrels, those assumptions are not valid. Particularly, the composition of a mathematical series describing an EM field at low frequencies changes when a highly conductive body is placed in the vicinity of sensors. Only if the mandrel material were perfectly conducting, would MFF be applicable.
One of the drawbacks of multifrequency acquisition is the time needed to acquire signals at a plurality of frequencies can be large. In addition, maintaining the transmitter circuits in tune for the plurality of frequencies may become a problem. In contrast, TEM methods require a shorter acquisition time and a single circuit. However, such methods typically use more complicated A/D conversion to handle small sampling intervals needed for TEM measurements. The present disclosure provides an improved method of processing TEM signals.